yard-documentation

- The README now includes instructions that describe how to setup a
  development environment.

- A new `Rakefile` can be used to generate API documentation with Yard
  and to run the test suite.

- A missing `lib/timelog.rb` file has been added and imports in tests
  have been updated to use it.

- All modules, classes, etc. have been updated with descriptions of
  parameters, return types, etc. using Yard-compatible documentation
  syntax.